1. Field of Invention
This invention relates to film cassettes containing self-developing or instant type film units which incorporate a dark slide to prevent exposure of the film units to light prior to the insertion of the cassette into an operative position in a camera.
2. Description of the Prior Art
Film units of the self-developing type are conventionally stacked in a film cassette for successive exposure in a photographic camera. The cassette includes a chamber having spaced walls including a forward wall forming a film exposure opening, a leading end wall having an opening through which exposed film units are withdrawn and a bottom wall. A spring platen is disposed within the cassette and is supported on the bottom wall thereof for urging film units stacked thereon toward the forward wall so that each successive film unit is placed in proper position in the exposure opening for exposure and for subsequent ejection through the leading end wall opening.
A light blocking mechanism known in the industry as a dark slide is initially located in the cassette, in place to block any light passing through the exposure opening until after the cassette is inserted into a camera. After the cassette is in the camera, the dark slide is ejected in a conventional fashion to allow the underlying film unit to be properly placed in the exposure opening, ready for its exposure upon the actuation of the camera's shutter mechanism.
Keeping the dark slide in position in the exposure opening prior to the placement of an underlying film unit in the proper exposure position within the camera is critical for the proper shielding of the underlying film unit from exposure to actinic light. Accordingly, the dark slide is essentially a rectangle having a greater length and width than the length and width of the exposure opening in the cassette. Problems occur when the cassette is jarred or impacted by forces during shipping or loading of the cassette into the camera. Sometimes an impact is of a magnitude that is sufficient to partially eject the dark slide from or misalign it in the cassette. When this happens the upper film unit is sometimes exposed along one edge or inadvertently removed with the dark slide which, of course, is undesirable to the camera operator.
A Japanese utility model application Laid-Open No. 01-75231, filed Nov. 9, 1987 entitled Film Pack, discloses a film cassette enclosing a stack of film units urged toward an exposure opening in the forward wall of the film cassette by a spring mechanism. A dark slide in the cassette intermediate the exposure opening and the first underlying film unit includes an upwardly extending boss projecting into an open aperture in the forward wall of the cassette. The boss and aperture combine to hold the dark slide against movement toward the film ejection slot during shipping and until the dark slide is mechanically ejected prior to film unit exposure. The dark slide is released for subsequent ejection by the use of a camera projection which passes downward through the open circular aperture in the forward wall of the cassette, when the cassette is fully inserted in the camera, to bias the boss of the dark slide out of frictional engagement with the aperture in the top of the film cassette. This boss would appear to work satisfactorily but it would require the redesign of internal camera structure.
U.S. Pat. No. 4,392,731 discloses in FIGS. 7 and 8 thereof a structure similar to that shown in the above-noted Japanese disclosure. The U.S. patent involves a film cassette having a stack of film units of the self-developing type mounted therein and biased forward by an underlying spring. The structure of significance in this patent is not the dark slide itself, but rather a filter which is permanently mounted in the film cassette by a boss and hole combination. Similar in structure to the Japanese utility model disclosure mentioned above, the combination is designed to maintain the filter permanently in place between the film units and the exposure aperture for each successive film unit ejection. Removal of the filter from the film cassette is accomplished by a plug passing through a hole in the forward wall of the film cassette to depress the boss on the filter against the upward bias of a spring platen. Ejection of the filter is accomplished by some separate mechanism.
The problem with the above-noted Japanese disclosure and U.S. patent is that both require additional moving parts in conventional cameras in order to allow for the removal of a dark slide or filter. Inherently, that requires a new internal camera structure before the apparatus can be used satisfactorily by a camera operator. Therefore, all existing cameras are eliminated. What is needed in the industry is an apparatus which may be used with any existing camera using self-developing type film whereby the dark slide is restrained from accidental displacement within the cassette and partial ejection therefrom as a result of impacts to the cassette which may occur during normal shipping, or loading of the cassette into a camera. It is axiomatic that someone will drop a film cassette during the film loading process and it is desirable to have some structure which prevents that type of impact from partially ejecting the dark slide from the cassette and thereby exposing the underlying film unit to light prior to normal film unit exposure within the camera.
Another problem with conventional dark slides is their relative thickness compared to the thickness of film units. Sometimes a thin dark slide allows the pick or film ejection means to engage the underlying surface of the uppermost film unit and tear its trailing upper surface or eject it with the dark slide.